1. Field of the Invention
This invention relates to a rubber composition and a pneumatic tire using the same, and more particularly to a rubber composition capable of using in a tire tread to improve a resistance to thermal fatigue without deteriorating a high steering stability on a dry road surface (dry gripping property).
2. Description of Related Art
The tire tread is required to have a high steering stability on a dry road surface (dry gripping property). In order to improve the dry gripping property, an amount of an oil component such as a resin, an aromatic oil, a liquid polymer or the like has hitherto been increased in a rubber composition for the tread. However, as the amount of the oil component is increased, a resistance to dynamically thermal fatigue is lowered and hence there is a problem that cracks are apt to be easily caused by heat generation and deformation of an interior of the tire during the running of the tire.
In regard to this problem, it is considered to use a thiuram compound, a metal salt of dithiocarbamic acid, a metal salt of dithiophosphoric acid or the like as a vulcanization accelerator capable of increasing monosulfide for mainly improving a thermally stability in crosslinking (JP-A-5-262916 and JP-A-2001-316527). However, as the compounding amount of such a compound is too large, the statically heat resistance is improved, but there is a problem that the fatigue resistance lowers and hence the resistance to thermal fatigue is not improved.
Also, it is considered to use a heat-resistant crosslinking agent such as phenylene bismaleimide, PK900 (N,N′-m-xylene-bis-citraconimide), sodium 1,6-hexamethylene dithiosulfate bihydrate or the like (JP-A-2000-301908). Such a compound is excellent in the reactivity to natural rubber and isoprene rubber, but is low in the reactivity to polybutadiene rubber and styrene-butadiene copolymer rubber having a high content of vinyl bond. Therefore, even if a greater amount of this compound alone is compounded with a rubber component having a high content of vinyl bond, there is a problem that the resistance to thermal fatigue is not improved.
As mentioned above, there is not yet provided a technique capable of largely improving the resistance to thermal fatigue in the tire while maintaining the high dry gripping property.